Giles Duley, centre, with his brother David and sister Sarah: '‘Without them I wouldn’t be here. It’s as simple as that. They pulled me through.’ Photograph: Martin Godwin for the Guardian

Giles Duley's work as a photographer often took him to dangerous places. So when he told his sister, Sarah De Schynkel, that he was off to Afghanistan to record the daily lives of US soldiers deployed there, she didn't give it a second thought. On the day he left, though, she remembers feeling sudden panic. "I texted him and said take care, won't you? And he texted back and said everything's fine, I'm just about to board the plane.

"Afterwards I thought, should I have tried to stop him? But it wouldn't have made a difference: who does anything their big sister tells them, after all?"

For the next three weeks, Sarah, 48, and her elder brother David, 64, hardly noticed their younger brother's absence. "He travelled all the time," says David. "We were used to it." Then he found himself answering the phone to an army liaison officer in Kandahar. "He said, 'I've got some bad news about Giles. He's been badly injured. I'm afraid he's lost his left leg.'

"Then he stopped for a second before continuing: 'And his right leg.' And then he stopped again, and then said: 'And his left arm.' He stopped again and I remember thinking, in a second he's going to tell me Giles has lost his right arm as well."

In fact, Giles's right arm, though very seriously damaged, was at least still attached to his body. Two years on, sitting in Sarah's kitchen in south-west London with the winter sun streaming in through the window, Giles, now 41, says surviving his catastrophic injuries came down to two factors. One was his right arm. "I lost absolutely everything I could afford to lose: any more, and I don't think life would have been liveable." The second was his brother and sister. "Without them I wouldn't be here. It's as simple as that. They pulled me through. They kept me going."

Before February 2011 and the day Giles, who was embedded with a US regiment, stepped on a landmine, you wouldn't have called the Duley siblings especially close. "We were like most families," says David. "We met up at Christmas, got together for special occasions. But we certainly didn't live in one another's pockets."

In fact, when David called Sarah to pass on the news from Kandahar, she knew immediately that something was wrong. "David wouldn't just phone me up like that," she says. "The second I heard his voice, I knew it was bad."

Over the next few days the family lived in a kind of limbo, as Giles was still in Afghanistan, too ill to be moved. Sarah and David told their father, Ray, and their other siblings – Carol, who lives in the US, and James. Everyone was desperately worried and determined to do all they could: but by the time the medical ambulance flew Giles back to Britain three days later, it was clear that Sarah and David were going to be the main support team. "Dad came to the hospital with us for Giles's arrival, and of course wanted to do everything he could to help. But he's in his 80s and it was clear that a lot of what was going to be needed would be too much for him."

Giles was admitted to the Queen Elizabeth Hospital in Birmingham, which specialises in battlefield injuries. Sarah will never forget seeing him there for the first time. "I was so nervous that my teeth were chattering," she says. "I kept thinking, I'm so scared of seeing him – what's he going to look like? And I couldn't help wondering, what's this going to mean? Is it going to be right for him to survive, given how terrible his injuries are?"

But it wasn't as frightening as she'd feared – and seeing Giles reassured her. "He had a sheet over him so we couldn't see the injuries at all. We could see his face, that it was Giles, that he was still himself and he was still with us," she remembers. "With that moment," says David, "we knew the battle was on. We knew we had to help Giles fight for his life."

The first few days were, in retrospect, almost a honeymoon period: Giles was conscious, chatting, even making jokes. But what none of them realised was the enormity of the struggle that lay ahead. Most soldiers who have been as badly injured as Giles was have died. What's more, they are half his age. "The doctors refused to speculate about what would happen, although they did say there were risks of infection," says David. "But he seemed so well that we thought he'd go from strength to strength."

Their father went home and David and Sarah moved into a flat near the hospital that was used by soldiers' families. Meanwhile, Giles was being put through one gruelling operation after another – sometimes, there wasn't even time to bring him round from the anaesthetic before he was taken back into theatre. Inevitably, complications set in. "He developed acute respiratory distress syndrome, which basically meant his lungs collapsed," says David. "Suddenly, he was very ill indeed. By this point, Sarah had gone home for a few days and I was there on my own. I said to the doctor, should I phone and tell the others to come? He said yes, that sounds a good idea. I asked, could he die tonight? And he said yes."

By the time Sarah got back to the hospital, Giles was in a coma-like condition. "He was just lying there, he couldn't respond. He was on a ventilator and his temperature was sky high ... then his kidneys starting to pack up as well and they put him on a dialysis machine."

David was told later that a visiting doctor, assessing patients in the intensive care unit, was told not to bother looking at Giles because he wasn't going to make it. There was speculation about whether he had been starved of oxygen at one point during a procedure, which might have caused brain damage. Another time, it wasn't clear whether he might always need to be on dialysis. "There was a point around that time when I thought, I don't know if this is worth it," says David. But one day, when David was at Giles's bedside alone, his brother started to respond. "It felt like he'd turned a corner," says David. "I thought, he's come back."

Giles remembers little of the weeks when he was so desperately ill , though he does recall snatches of drifting in and out of consciousness. "I was aware that Sarah and David were always there. At one point I remember saying, why are you both here all the time? They said, we've moved here to be near you. I thought: this must be bad, then, if they've done that."

One of the first things Giles had said to Sarah after he arrived in Birmingham was that because he still had an arm he could still be a photographer. All three of them held on to that, knowing how important a part of Giles's life his career had always been.

For Sarah, one of Giles's photographs came to symbolise their experience. "When I was leaving home to go to Birmingham that first time, I thought, what can I take? What's going to help me through this? In the end, I took a photograph of our mother, Flora, who died four years ago. And I also took a postcard image of a photograph Giles took on a trip to Bangladesh a few years ago. The picture shows a girl, aged about 12, looking after her little sister, who's about three. She's holding on to her really tightly. I realised that was what I was going to have to do: I was going to have to look after my little brother and to hold him really tightly until he was recovered."

Being next of kin isn't a role that often falls to siblings; most often, it's parents or spouses who make the vital decisions when someone is gravely ill. "It did seem strange, having to speak on Giles's behalf and having to be the conduit to all his friends because he was too ill to see anyone except family for a long time," says Sarah.

One of the people she had to contact was Giles's girlfriend, Jen. "We didn't know one another at all before it happened," says Sarah. "But it became clear that this was a big relationship, so it was odd having to be their go-between."

As Giles got better, Sarah and David were able to retreat back gently into their own lives (though not before Giles had spent several months convalescing at Sarah's home). "When I was making decisions about my future, it helped that Sarah and David weren't going to be affected by them the way a parent or spouse would have been," says Giles. "They were absolutely around when it mattered. But as I recovered they weren't in my way, they didn't complicate things for me."

It seems almost trite to say that the experience has brought the three of them closer together: today, they finish one another's sentences, understand one another's need to tell their part of the story, laugh at shared tales of funny moments in intensive care or rehab. But, says David, they are not exceptional. "What we did was what anyone would do for their brother," he says. "When he needed us, we were there."

• Walking Wounded: Return to the Frontline – a documentary to be shown on Channel 4 at 10pm on 21 February – follows Giles Duley back to Afghanistan to report on what happens to Afghans who, like him, became amputees because of the war

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